Synchronization of image acquisition in multiple image sensors with a synchronization clock signal

ABSTRACT

A multiple image sensor image acquisition system includes a clock control unit to generate a synchronization clock signal. The synchronization clock signal has a prolonged constant cycle during which the synchronization clock signal is held at a constant level for a period of time corresponding to multiple clock cycles. A first image sensor is coupled with the clock control unit to receive the synchronization clock signal and has a first synchronization unit that is operable to synchronize operation for the first image sensor based on detection of an end of the prolonged constant cycle. A second image sensor is coupled with the clock control unit to receive the synchronization clock signal and has a second synchronization unit that is operable to synchronize operation for the second image sensor based on detection of the end of the prolonged constant cycle. The image sensors are synchronized operationally.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 13/622,976, filed Sep. 19, 2012, U.S. patent application Ser. No. 13/440,833, filed Apr. 5, 2012, and U.S. patent application Ser. No. 13/440,846, filed Apr. 5, 2012. U.S. patent application Ser. Nos. 13/622,976, 13/440,833, and 13/440,846 are incorporated herein by reference.

BACKGROUND

1. Field

Embodiments relate to acquiring images with pixel arrays. In particular, embodiments relate to acquiring images with complementary metal oxide semiconductor (CMOS) pixel arrays.

2. Background Information

FIG. 1 is a block diagram of a known image sensor package 100 that includes an image sensor 101 having a complementary metal oxide semiconductor (CMOS) pixel array 102, control circuitry 103, and readout circuitry 104. Commonly, the CMOS pixel array, the readout circuitry, and the control circuitry are monolithically integrated on a single die or other substrate. The image sensor package provides interconnections (not shown), such as pads, to connect the image sensor with an external signaling medium (e.g., circuitry of a digital camera or other system having the image sensor package).

The CMOS pixel array 102 includes a two-dimensional array of CMOS pixels (e.g., pixels P1, P2, . . . Pn). As illustrated, the pixels are arranged in rows (e.g., rows R1 through Ry) and columns (e.g., column C1 through Cx). Commonly there may be anywhere from hundreds to many thousands each of rows and columns of pixels. During image acquisition, the pixels may acquire image data (e.g., photogenerated electrical charges). The image data from all of the pixels may be used to construct an image as is known in the art.

The control circuitry 103 and the readout circuitry 104 are coupled with the CMOS pixel array. The control circuitry is operable to apply electrical signals to the CMOS pixel array to control or assist with controlling aspects of image acquisition. The readout circuitry is operable to read out the image data from the pixels. Commonly, the readout circuitry may read out image data from a single row of pixels at a time along column readout lines 105. The column readout lines are also sometimes referred to as bitlines. The readout circuitry may potentially include amplification circuitry, analog-to-digital conversion (ADC) circuitry, gain control circuitry, or the like. The image data signals 106 may be provided from the readout circuitry to an external signaling medium (e.g., circuitry of a digital camera or other systems having the image sensor package).

The CMOS pixel array 102 commonly uses an electrical rolling shutter. During the image acquisition process, the CMOS pixel array may be exposed to constant and/or continuous light 107 and the electrical rolling shutter may control the amount of exposure that the pixels of the CMOS pixel array are subjected to under the constant/continuous light. For example, in an electrical rolling shutter each row of pixels may be exposed to light during a different period of time in a rolling or sequential fashion. For example, for each acquired image the rows of pixels may be exposed to light sequentially row-by-row from the first row R1 to the last row Ry. As shown, clock signals 108 and rolling shutter image acquisition control signals 109 may be provided to the control circuitry from an external signaling medium (e.g., circuitry of a digital camera or other systems having the image sensor package). The control circuitry may apply electrical signals to the CMOS pixel array based on the received clock and control signals to implement the electrical rolling shutter operations.

FIG. 2 is a circuit diagram illustrating known pixel circuitry 202 for two four-transistor (4T) pixels P1 and P2 of a CMOS pixel array. The pixels P1 and P2 are arranged in two rows and one column and time share a column readout line 205. By way of example, the pixel circuitry may be implemented in the pixels P1 and P2 of the CMOS pixel array 102 of FIG. 1.

Each of the pixels includes a photodiode PD, a transfer transistor T1, a reset transistor T2, an amplifier or source-follower SF transistor T3, a row select transistor T4, and a floating diffusion node FD. Within each pixel, the photodiode is coupled to the floating diffusion node FD by the intervening transfer transistor T1. A transfer signal TX asserted on the gate of the transfer transistor T1 activates the transfer transistor T1. The floating diffusion node FD may represent a circuit node to receive and hold a charge. The reset transistor T2 is coupled between a supply voltage VDD and the floating diffusion node FD. A reset signal RST asserted on the gate of the reset transistor T2 activates the reset transistor T2. The source-follower SF transistor T3 is coupled between a voltage supply VDD and the row select transistor T4. The source-follower SF transistor T3 has a gate coupled to the floating diffusion node FD and a channel selectively coupled to the column readout line 205 through the row select transistor T4. The source-follower SF transistor T3 is coupled to the column readout line when a row select signal SEL is asserted on the gate of the row select transistor T4. The row select transistor T4 selectively couples the output of the pixel to the column readout line 205 when the row select signal SEL is applied to the gate of the row select transistor T4.

FIG. 3 is a plot illustrating timing of known electrical rolling shutter image acquisition control signals that are suitable for implementing an electrical rolling shutter for two rows of a pixel array. Electrical rolling shutter image acquisition control signals are plotted for each of two rows, namely row R1 and row R2, on the vertical axis. Progression of time is plotted from left to right on the horizontal axis. To facilitate description, the electrical rolling shutter image acquisition control signals are described in conjunction with the components and signals of the pixels P1 and P2 of FIG. 2.

Referring to the electrical rolling shutter image acquisition control signals for row R1, the gate of the reset transistor T2 is initially activated by application of a reset signal RST at time t1. While the gate of the reset transistor T2 is activated, a gate of the transfer transistor T1 is pulsed with a transfer signal TX between times t2 and t3. As a result, the photodiode PD and the floating diffusion node FD are reset to the supply voltage VDD. The transfer signal TX is de-asserted at time t3. After the reset, the production and accumulation of photo-generated charges in the photodiode PD begins. The production and accumulation of photo-generated charges in the photodiode PD is also referred to herein as integration. As previously mentioned, there is typically constant/continuous light to expose the photodiode PD throughout the integration. The photodiode PD is operable to generate charges (e.g., photogenerated electrons or holes) in response to such light. As photogenerated charges, for example electrons accumulate on the photodiode PD, its voltage may decrease, since electrons are negative charge carriers (or in the case of photogenerated charges being holes, the voltage may increase accordingly). The amount of voltage or charges accumulated on the photodiode PD may be indicative of the amount and/or intensity of the light incident on the photodiode PD during the exposure period, and may represent image data. For constant intensity light, the longer the exposure period, which is determined by the particular electrical rolling shutter, the more the accumulation of charges.

The reset signal RST may be de-asserted at time t4 to electrically isolate the floating diffusion node FD. A select signal SEL is asserted to the gate of the row select transistor T4 at time t5. This prepares the row R1 of pixels for readout. The gate of the transfer transistor T1 is activated by application of the transfer signal TX between times t6 and t7. This causes the transfer transistor T1 to transfer the photo-generated charges (e.g., electrons) accumulated in the photodiode PD to the floating diffusion node FD. The charge transfer may cause the voltage of the floating diffusion node FD to drop from the supply voltage VDD to a second voltage that is indicative of the image data (e.g., photogenerated electrons accumulated on the photodiode PD during the exposure period). Integration ends upon the finish of the charge transfer. The floating diffusion node FD is coupled to control the gate of the source-follower SF transistor T3. The floating diffusion node FD is presented to the gate of the source follower SF transistor T3. Source-follower SF transistor T3 operates to provide a high impedance connection to the floating diffusion node FD. The source follower SF transistor T3 amplifies the photogenerated charge signal, which is read out to column readout line 205 by row select transistor T4. The row select signal SEL applied to the row select transistor T4 is deactivated at time t8. This completes the readout operation.

As shown, in an electrical rolling shutter, the signals for row R2 each start a predetermined time after the corresponding signals for row R1. That is, each control signal (i.e., RST, TX and SEL) for row R2 is asserted after its counterpart control signal for row R1 has been asserted. The first row R1 is reset, integration is initiated, and then the first row R1 is readout generally a predetermined time after reset. Similarly, the second row R2 may be reset a predetermined time after resetting the first row R1, integration in the second row R2 may be initiated, and then the second row R2 may be readout after the first row R1 has been readout. Notice that integration for row R2 occurs after integration for row R1. It is common that the integration for row R2 starts during the time that the integration for row R1 is taking place. It is noted that the signals in the illustration are not drawn precisely to scale. Such a process may be repeated for all of the other rows of pixels of a CMOS pixel array, sequentially, row-by-row, from the first row R1 to the last row Ry, for each acquired image.

FIG. 4 is a block diagram of a known reset-readout block 410 that represents the reset and readout operations performed when acquiring a single image frame using an electrical rolling shutter. Progression of time 411 is plotted on the vertical axis from top to bottom. The reset-readout block has the shape of a parallelogram. A left vertical side of the parallelogram represents a reset line 412. The reset line is bounded between resetting of the first row R1 (at the top left corner of the parallelogram) through the resetting of the last row Ry (at the bottom left corner of the parallelogram). The intermediate rows between R1 and Ry are reset sequentially row-by-row or one-by-one after the first row R1 though the last row Ry. A right vertical side of the parallelogram represents a readout line 413. The readout line is bounded between readout of the first row R1 (at the top right corner of the parallelogram) through readout of the last row Ry (at the bottom right corner of the parallelogram). The intermediate rows between R1 and Ry are readout sequentially row-by-row or one-by-one after the first row R1 though the last row Ry. The resetting of rows R1 to Ry typically takes the same amount of time as the readout of rows R1 to Ry.

Within an image frame, each row is initially reset and then subsequently read out after a generally predetermined time. The time between the resetting of the row, and readout of that row, represents the exposure period during which the pixels of that row are configured to perform photoelectric charge production and accumulation (i.e., integration). As illustrated by arrow 407, there is typically constant/continuous illumination from at least the resetting of the first row R1 through the readout of the last row Ry. Notice also that the readout of the first row R1 typically begins well before the resetting of the last row Ry. This is typically done to help reduce the overall amount of time needed to acquire an image frame.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments. In the drawings:

FIG. 1 is a block diagram of a known image sensor package that includes an image sensor having a complementary metal oxide semiconductor (CMOS) pixel array, control circuitry, and readout circuitry.

FIG. 2 is a circuit diagram illustrating known pixel circuitry for two four-transistor (4T) pixels P1 and P2 of a CMOS pixel array.

FIG. 3 is a plot illustrating timing of known electrical rolling shutter image acquisition control signals that are suitable for implementing an electrical rolling shutter for two rows of a pixel array.

FIG. 4 is a block diagram of a known reset-readout block that represents the reset and readout operations performed when acquiring a single image frame using an electrical rolling shutter.

FIG. 5 is a block diagram of an embodiment of an endoscope video image acquisition system.

FIG. 6 is a block diagram conceptually illustrating an example of an image distortion that may result when a moving CMOS pixel array using an electrical rolling shutter acquires an image of a stationary object.

FIG. 7 is a block flow diagram of an embodiment of a method of acquiring global shutter-type video images with an endoscope probe having a CMOS pixel array that uses an electrical rolling shutter.

FIG. 8 is a block diagram illustrating an embodiment of acquiring a sequence of global shutter-type video images using a CMOS pixel array using an electrical rolling shutter in a dark environment.

FIG. 9 is a block flow diagram of an embodiment of a method of acquiring a sequence of global shutter-type video images with a CMOS pixel array using an electrical rolling shutter in a dark environment.

FIG. 10 is a block diagram of reset-readout blocks for consecutive video image frames that illustrate an “illumination scheme A” embodiment in which illumination with light from the light source occurs over potentially the entire and/or over potentially any portion of a vertical blanking period.

FIG. 11 is a block diagram of reset-readout blocks for consecutive video image frames that illustrate an “illumination scheme B” embodiment in which illumination with light from the light source occurs over potentially an entire and/or potentially any portion of only a post-reset portion of a vertical blanking period.

FIG. 12 is a block diagram of an embodiment of a video image acquisition system that is operable to acquire global shutter type video images with a CMOS pixel array that is to use an electrical rolling shutter.

FIG. 13 is a block diagram of an embodiment of a portion of a video image acquisition system having an automatic exposure control unit.

FIG. 14 illustrates conventional un-prolonged vertical blanking periods, an embodiment of prolonged vertical blanking periods, and an embodiment of an approach for prolonging the prolonged vertical blanking periods by holding a clock signal at a constant level during the prolonged vertical blanking periods.

FIG. 15 illustrates an embodiment to reduce light duration period by providing light in pulses.

FIG. 16 is a block diagram of an embodiment of multiple image sensor video image acquisition system that is operable to use a synchronization clock signal to synchronize a first image sensor and a second image sensor.

FIG. 17A is a diagram illustrating an example embodiment of a prolonged high synchronization clock signal.

FIG. 17B is a diagram illustrating an example embodiment of a prolonged low synchronization clock signal.

FIG. 18 is a block diagram of an embodiment of an image sensor having an example of a suitable set of pads or other electrical contact structures.

FIG. 19 is a block diagram of an embodiment of a multiple image sensor video image acquisition system that is operable to acquire global shutter-type video images with a CMOS pixel array that is to use an electrical rolling shutter.

FIGS. 20A-D show embodiments of endoscopes that include light sources and CMOS pixel arrays that are to use electrical rolling shutters.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth (e.g., specific endoscope systems, specific methods, specific orders of operations, specific timing of illumination, specific rolling shutter image acquisition control signals, specific reset-readout blocks, specific integration/division options for components, etc.) However, embodiments may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail to avoid obscuring the understanding of the description.

FIG. 5 is a block diagram of an embodiment of an endoscope video image acquisition system 515. The endoscope video image acquisition system includes an endoscope base station 516, an endoscope probe 517, and one or more connectors 518 to connect or otherwise couple the endoscope probe with the endoscope base station. The endoscope probe and the endoscope base station may represent any of the various different types of endoscope probes and endoscope base stations known in the arts. The scope of the invention is not limited to any known type of endoscope probe or endoscope base station.

The endoscope probe 517 often may be a relatively small device that is sized and shaped for insertion into a subject of endoscope examination through various ways known in the art. The endoscope probe includes a CMOS pixel array 502 that is to use an electrical rolling shutter to acquire images. In some embodiments, the CMOS pixel array 502 may be similar to, or the same as, the CMOS pixel array 102 of FIG. 1. Alternatively, a different CMOS pixel array 502 may be used. In some embodiments, the CMOS pixel array 502 may have four-transistor (4T) pixels similar to, or the same as, those shown and described for FIG. 2. Alternatively, different types of pixels may be used.

The endoscope base station 516 often may include an enclosure or housing having therein various different types of components to support the operation of the endoscope probe including operations associated with image acquisition. Examples of different types of components that may be included in the endoscope base station in some embodiments include, but are not limited to, a power supply, clock circuitry, control circuitry, image processing logic, an optional light source (e.g., one or more light emitting diodes (LEDs), lasers, coherent light sources, lamps, etc.), one or more memories, one or more processors, etc. The scope of the invention is not limited to any particular known set of components. The connector(s), such as, for example, one or more flexible cables, may be connected and disconnected between a connector interface 519 of the endoscope base station and a connector interface 520 of the endoscope probe. The connector(s) may house wires or other electrical signaling paths and optical fibers or other optical signaling paths.

The endoscope probe 517 may be used in a dark environment inside the subject of endoscope examination, such as a patient 521. In various different embodiments, the endoscope probe may be inserted into the patient through a native body orifice or opening (e.g., a throat, nose, anus, etc.), through a man-made opening into a body cavity or lumen (e.g., inserted through a surgical opening into a chest, other body cavity, blood vessel, etc.). Such regions within the patient represent dark environments in which there is typically no, or at least insufficient, natural or ambient light, to acquire meaningful images (e.g., images acquired would typically be too dark and/or of insufficient quality to be practically useful for diagnosis or examination). As shown, in some embodiments the endoscope base station may have a light source 522 to provide light to the endoscope probe to help illuminate the dark environment. Alternatively, a light source separate from the endoscope base station may be used (not shown in FIG. 5). As one example, a light source within the endoscope probe (e.g., one or more light emitting devices) may be used. As another example, a stand-alone separate light source not within an enclosure or housing of the endoscope base station may be used. Typically, other than the light provided by the light source, the only natural or ambient light in the dark environment inside the patient 521 is the typically very small amount of light that may get in through the orifice or surgical opening from the external environment where the patient resides.

Once inserted into the patient, the endoscope probe may be navigated, advanced, or otherwise moved within the patient. For example, the endoscope probe may be moved within the patient toward a desired destination (e.g., a region, path, or anatomical feature that is to be examined and/or treated). While the endoscope probe is being moved within the patient, a sequence of video images may be acquired with the CMOS pixel array 502 of the endoscope probe. Without limitation, the video images may potentially be used to help navigate or advance the endoscope probe. Moreover, the video images may potentially also be used for medical examination or diagnosis. In any event, it is generally desirable for the video images to be of sufficiently high quality and to lack significant image artifacts or distortions.

As previously mentioned, the CMOS pixel array is to use an electrical rolling shutter to acquire the video images. One challenge is that image artifacts or distortions may be introduced into the video images that are acquired by using the electrical rolling shutter when there is relative movement between the CMOS pixel array and objects or environments being imaged (e.g., when the CMOS pixel array is being moved within a relatively stationary patient). The distortions and/or artifacts tend to occur due in large part to the different rows of pixels integrating at different times according to the electrical rolling shutter. The movement occurs during the time over which the different rows of pixels integrate within a given image frame. For example, the CMOS pixel array may move relative to a generally stationary object being imaged between the time when the first row of pixels photo-generates and accumulates charge and the time when the last row of pixels photo-generates and accumulates charge within the same image frame such that the first and last rows of pixels may image the object in motion when it is located at different positions. This may cause image artifacts or distortions to appear in the image.

FIG. 6 is a block diagram conceptually illustrating an example of an image distortion that may result when a moving CMOS pixel array 602 using an electrical rolling shutter acquires an image 623 of a stationary object 621. The CMOS pixel array 602 is moving from left to right, with respect to the stationary object 621. Alternatively speaking, with respect to the CMOS pixel array 602, the object 621 may be viewed as “moving” from right to left. The illustrated stationary object 621 is a circle. The image acquired with the moving CMOS pixel array using the electrical rolling shutter has a distorted representation of the circle. As shown, the distorted representation of the circle is an oval 624. The top of the oval slants or leans in the direction of movement of the CMOS pixel array. Alternatively speaking, the bottom of the oval slants or leans in the direction of the “movement” of the object 621 relative to the CMOS pixel array 602. As mentioned, this image distortion is largely due to the rows of pixels of the CMOS pixel array integrating at different times while movement is occurring.

One way to avoid, or at least reduce, such image distortions is to use an electrical global shutter. In an electrical global shutter, all of the rows of pixels of the pixel array will integrate simultaneously (e.g., all rows start integration at the same time and all rows end integration at the same time) instead of integrating sequentially row-by-row as in an electrical rolling shutter. In an electrical global shutter, relative movement between the pixel array and the object being imaged will not produce the type of image distortion shown in FIG. 6. Electrical global shutters are common to charge coupled device (CCD) pixel arrays and may also be implemented in CMOS pixel arrays. However, implementing an electrical global shutter in a CMOS pixel array tends to have certain drawbacks. For one thing, one or more additional transistors are generally incorporated in each pixel of the CMOS pixel array in order to help implement the electrical global shutter. For example, each CMOS image sensor pixel may include a reset transistor that receives a global reset signal to reset all photodetectors at the same time to ensure that all the pixels start integration at the same time, and a storage transistor to transfer the photo charges from the photodetectors to floating diffusion at the same time and hold the charges until they are read out later. Such additional transistors tend to increase the size of the CMOS pixel array, which is generally undesirable especially for various endoscope applications. Moreover, these additional transistors also tend to increase the overall manufacturing cost of the CMOS pixel array.

Referring again to FIG. 5, the endoscope base station 516 includes an apparatus 525 that is operable to acquire global shutter-type video images with the CMOS pixel array 502 having the electrical rolling shutter. As used herein global shutter-type video images are those generated with all, or at least a vast majority, of the rows of pixels of the CMOS pixel array integrating concurrently over a same period of time. As used herein, at least a vast majority of the rows of pixels of the CMOS pixel array means at least 90% of the rows of pixels of the CMOS pixel array.

As shown, the apparatus and/or the endoscope base station may provide an embodiment of video image acquisition control signals 526 to the CMOS pixel array and/or the endoscope probe over the connector(s) to cause the CMOS pixel array and/or the endoscope probe to acquire the global shutter-type video images. In some embodiments, the video image acquisition control signals 526 may include electrical rolling shutter video image acquisition control signals. For example, in some embodiments, the signals for each video image may be operable to reset each row of pixels of the CMOS pixel array sequentially, and one row at a time, from a first row to a last row, and to read each row of pixels of the CMOS pixel array sequentially, and one row at a time, from the first row to the last row. In some embodiments, the signals may define, between each pair of consecutive video images, a vertical blanking period that occurs in time between the reading of the last row of pixels of a previous video image and the reading of the first row of pixels of a subsequent video image. In some embodiments, the video image acquisition control signals are such that, within each video image frame, the resetting of the last row is controlled to be performed before the reading of the first row.

As shown, the apparatus 525 may also provide light strobing control 527 to the light source 522. The light strobing control could alternatively be provided to a light source located outside of the endoscope base station as previously described (e.g., within the endoscope probe or as a separate stand-alone light source outside of the endoscope base station housing/enclosure). The light strobing control may control the light source to provide strobed light 528 to illuminate the dark environment inside of the patient 521. As used herein strobed light refers to light that is intermittently turned on and off, or is turned up or dimmed down, multiple or many times in succession. In some embodiments, the strobed light is only turned on or turned up during at least a portion of each vertical blanking period that occurs between sequential video image frames, and is turned off or dimmed down within each of the video image frames during the periods of time while the rows of pixels are being read out. For example, within a pair of sequential video image frames, the strobed light may be off between the reading/readout of the first and last rows of pixels during an earlier video image frame, may be off between the reading/readout of the first and last rows of pixels during a later video image frame, and may be on during at least a portion of a vertical blanking period between the reading/readout of a last row of pixels of the earlier video image frame and a reading/readout of a first row of pixels of the later video image frame.

As will be explained further below, the strobed light 528 together with the otherwise dark environment inside the patient 521 may effectively cause or result in all, or at least a vast majority, of the rows of pixels of the CMOS pixel array 502 to integrate concurrently over a same period of time, even when an electrical rolling shutter is being used for image acquisition, to allow global shutter-type video images to be acquired. Advantageously, this may help to eliminate, or at least reduce, the amount of image artifacts or distortion that would otherwise tend to occur due to relative movement between the CMOS pixel array and the objects or things that are being imaged. As shown, image data with reduced distortion 529 may be provided from the endoscope probe to the endoscope base station.

FIG. 7 is a block flow diagram of an embodiment of a method 730 of acquiring global shutter-type video images with an endoscope probe having a CMOS pixel array that uses an electrical rolling shutter. At block 731, the endoscope probe having the CMOS pixel array that is to use the electrical rolling shutter is inserted into a patient. The endoscope probe is moved within the patient, at block 732. A sequence of video images are acquired with the CMOS pixel array, while moving the endoscope probe within the patient, using the electrical rolling shutter, at block 733. At block 734, light from a light source is strobed so that the light is on during at least a portion of each vertical blanking period between consecutive video images and off within each video image while reading out rows of pixels of the CMOS pixel array. While shown sequentially, blocks 733 and 734 may be carried out concurrently or in parallel to acquire global shutter-type video images with an endoscope probe having a CMOS pixel array that uses an electrical rolling shutter.

FIG. 8 is a block diagram illustrating an embodiment of acquiring a sequence of global shutter-type video images using a CMOS pixel array using an electrical rolling shutter in a dark environment 835. Within a first video image frame 835-1: (1) a light source is turned off (or dimmed down); (2) no (or very little) charge generation or accumulation occurs since the environment is dark; and (3) a readout of charge accumulated by the CMOS pixel array during an immediately prior vertical blanking period (not shown) occurs row by row from top to bottom. Within a first vertical blanking period, which is subsequent to the first video image frame 836-1: (4) the light source is turned on (or turned up) for at least a portion of the first vertical blanking period; and (5) photoelectric charge is generated (or generated in large amount) and accumulated while the light source is turned on.

Within a second video image frame 835-2, which is subsequent to the first vertical blanking period: (6) the light source is again turned off (or dimmed down); (7) no (or very little) charge generation or accumulation occurs since the environment is dark; and (8) a readout of charge accumulated by the CMOS pixel array during the immediately prior first vertical blanking period 836-1 occurs row by row from top to bottom. Within a second vertical blanking period 836-2, which is subsequent to the second video image frame: (9) the light source is again turned on (or turned up) for at least a portion of the second vertical blanking period; and (10) photoelectric charge is generated (or generated in large amount) and accumulated while the light source is turned on. A subsequent video image frame (not shown) may read out the charge accumulated during the second vertical blanking period. Earlier and subsequent video image frames, and their associated vertical blanking periods, may be similar to those shown.

FIG. 9 is a block flow diagram of another embodiment showing a method 937 of acquiring a sequence of global shutter-type video images with a CMOS pixel array using an electrical rolling shutter in a dark environment. The CMOS pixel array is introduced into the dark environment at block 938. In some embodiments, an endoscope probe having the CMOS pixel array may be inserted into a patient. Alternatively, in other embodiments, a boroscope, hydraulic pig, or other inspection device having a CMOS pixel array may be inserted into an engine, a tube, a pipeline, or some other dark environment. In some embodiments, the dark environment has substantially no ambient light (e.g., an insufficient amount of ambient light to acquire images that are of sufficient quality to be practically useful). In some embodiments, the dark environment has a darkness value as indicated by a luminance value of less than 1 nit (candela per m²). An even darker environment may have a substantially lower luminance value, for example, 10⁻⁴ nit.

At blocks 939-942, video image frames of the dark environment are acquired with the CMOS pixel array using an electrical rolling shutter. At block 939, during a first video image frame, each row of pixels of the CMOS pixel array is reset sequentially, and one row at a time, from a first row to a last row. Sometime often after the reset of the last row of the CMOS pixel array, integration occurs. See block 941 below, as well as FIGS. 10 and 11, for more detail. Each row of pixels of the CMOS pixel array is then read out sequentially, and one row at a time, from the first row to the last row. Sometime before the readout of the first row of the CMOS pixel array, integration ends. See block 942 below, as well as FIGS. 10 and 11, for more detail. In some embodiments, the resetting of the last row during the first image frame is performed before the reading of the first row during the first image frame.

At block 940, during a second video image frame, each row of pixels of the CMOS pixel array is reset sequentially, and one row at a time, from the first row to the last row. Similar to block 939 above, sometime after the reset of the last row of the CMOS pixel array, integration occurs. Each row of pixels of the CMOS pixel array is then read sequentially, and one row at a time, from the first row to the last row. Similar to block 939 above, sometime before the readout of the first row of the CMOS pixel array, integration ends. In some embodiments, the resetting of the last row during the first image frame is performed before the reading of the first row during the first image frame.

A light source is controlled to substantially illuminate the otherwise dark environment during at least a portion of a vertical blanking period, at block 941. The vertical blanking period is the period between the reading of the last row of pixels during the first video image frame, and the reading of the first row of pixels during the second video image frame. For example, in some embodiments, substantial illumination may include switching on (or turning up) a light source (e.g., switching on or turning up power to one or more LEDs, one or more lasers, one or more coherent light sources, one or more lamps, one or more bulbs, or one or more other light emitting devices). Alternatively, in other embodiments, rather than switching on the light source, a shutter may be opened to allow passage of the light to the dark environment, the light may be reflected, diverted, directed, or otherwise mechanically and/or electrically controlled to be introduced into the dark environment. To substantially illuminate an environment means to provide a luminance that is at least five times more than the luminance when the environment is substantially not illuminated. For example, when an environment is substantially illuminated, the luminance provided is 10 to 100 times more than the luminance when the environment is not substantially illuminated. In another example of substantial illuminating an environment, the light source may be providing a luminous power, i.e., a luminous flux, of approximately 10 to 50 lumen (candela per steradian).

At block 942, the light source is controlled to substantially not illuminate the dark environment between the reading of the first and last rows of pixels during the first video image frame and between the reading of the first and last rows of pixels during the second video image frame. For example, in some embodiments, this may include switching off or turning down a light source. Alternatively, in other embodiments, a shutter may be closed to block passage of the light to the dark environment, the light may be reflected, diverted, or directed away from the dark environment, or otherwise mechanically and/or electrically controlled to not be introduced into the dark environment.

While blocks 939, 940, 941 and 942 are shown sequential, it is to be appreciated that block 941 may occur generally between but potentially partly concurrently with blocks 939 and 940. Moreover, block 942 may occur generally concurrently with block 939 and 940.

In some embodiments, blocks 941 and 942 may include controlling the light source to provide strobed light. The strobed light may substantially illuminate the otherwise dark environment only within sequential vertical blanking periods, but not substantially illuminate the dark environment within video image frames during readout of rows of pixels between the vertical blanking periods.

In some embodiments, substantial illumination with light from the light source occurs only during at least a portion of each of the (or at least some of the) vertical blanking periods. For purposes of illustration, two different possible illumination schemes will be described in detail below. First an “illumination scheme A” embodiment will be described and then later below an “illumination scheme B” embodiment will be described.

FIG. 10 is a block diagram of reset-readout blocks for consecutive video image frames that illustrate an “illumination scheme A” embodiment in which substantial illumination with light from the light source occurs over potentially the entire and/or over potentially any portion of a vertical blanking period 1036. The substantial illumination starts immediately or sometime after start of the vertical blanking period (e.g., immediately or sometime after the last row of pixels is readout in an earlier video image frame) and ends at or sometime before the end of the vertical blanking period (e.g., at or sometime before the first row of pixels is readout during a later video image frame).

As shown on the left side of FIG. 10, progression of time is plotted on the downward-pointing vertical axis from top to bottom. A first reset-readout block 1035-1 for a first, earlier video image frame and a second reset-readout block 1035-2 for a second, later video image frame are shown. Each of the reset-readout blocks has the shape of a parallelogram. A left vertical side of each parallelogram represents a reset line 1045. Each reset line is bounded by resetting the first row R1 (at the top left corner of the parallelogram) through the last row Ry (at the bottom left corner of the parallelogram). Intermediate rows are reset row-by-row or one-by-one sequentially between the first and last rows. A right vertical side of each parallelogram represents a readout line 1046. Each readout line is bounded by readout of the first row R1 (at the top right corner of the parallelogram) through the last row Ry (at the bottom right corner of the parallelogram). Intermediate rows are read out row-by-row or one-by-one sequentially between the first and last rows. Each row is reset before it is subsequently read out at a predetermined later time, with integration occurring after reset and before readout. Resetting from rows R1 through Ry typically takes the same amount of time as readout from rows R1 through Ry.

As shown in FIG. 10, the reset-readout blocks 1035, in some embodiments, the readout of the first row R1 of each video image frame begins after (i.e., is horizontally below) the resetting of the last row Ry within that video image frame. This is different from a prior art reset-readout block 410 as shown in FIG. 4. Referring back to the reset-readout block of FIG. 4, the readout of the first row R1 (at the top right corner of reset-readout block 410 parallelogram) begins well before the resetting of the last row Ry (at the lower left corner of the reset-readout block 410 parallelogram). The reset-readout scheme shown in FIG. 4 reduces the vertical length of the reset-readout block 410, and is done so as to reduce the overall amount of time needed to acquire an image frame and/or to achieve a high video image frame rate, which is typically desired. However, as shown in FIGS. 10 and 11, in some embodiments, resetting the last row Ry of each video image frame before the readout of the first row R1 within the same video image frame offers a potential advantage that all or at least a vast majority of the rows of pixels have been reset and are made ready to begin integration over the same period of time with the strobed illumination light before readout begins, as disclosed herein. This integration timing results in a substantial global shutter-type effect, which is desirable because it substantially overcomes the image distortion caused by the relative motion between the CMOS pixel array and the object that is being imaged.

Notice in FIG. 10 that the reset-readout blocks 1035 have a prolonged integration period, i.e., the period between resetting a given row until reading out the same given row (the vertical distance between the top left corner and the top right corner of the reset-readout blocks 1035 parallelograms), in contrast with the reset-readout block of FIG. 4. Visually, the reset-readout blocks 1035 with a prolonged integration period in FIG. 10 appear to be more vertically elongated than the reset-readout block 410 with a less-prolonged integration period in FIG. 4. The prolonged integration periods may be achieved by resetting a given row (for a second image frame) immediately or relatively soon after reading out that same given row (for a first image frame), such that the readout-to-reset time for the given row is relatively small. Generally, a given imaging cycle for a given row, from resetting the given row in the current imaging frame until resetting the same given row again in the next imaging frame, has fixed time duration. This fixed time duration is the sum of the reset-to-readout time period (e.g., a first vertical distance between the top left corner and the top right corner of the reset-readout block 1035-1 parallelogram) and the readout-to-reset time period (a second vertical distance between the top right corner of the reset-readout block 1035-1 parallelogram and the top left corner of the reset-readout block 1035-2 parallelogram). Reducing or minimizing the readout-to-reset time period (the second vertical distance) helps to increase or maximize the reset-to-readout time period (the first vertical distance). In other words, the integration time is increased or maximized. In some embodiments, after reading out the first row of pixels for the first image frame, the same first row of pixels will be reset for the second image frame within a period of time that is sufficiently short so as to read out no more than about an initial 5% of the rows of pixels of the CMOS pixel array. This helps to prolong the reset-to-readout integration period. However, in other embodiments such prolonged reset-to-readout integration periods are not required.

In the “illumination scheme A” embodiment, illumination with light from the light source occurs over potentially the entire and/or over potentially any portion of the vertical blanking period 1036. In this scheme, illumination with light from the light source does not extend outside the vertical blanking period. For example, the illumination does not occur during the readout of the rows R1-Ry of pixels in either of the bounding video image frames. As shown, within the duration of the readout lines (i.e., the right lines of the two parallelograms 1035) of the bounding video image frames there is a dark environment 1047 with no illumination from the light source. By contrast, in FIG. 4 there is constant/continuous illumination 407 (as represented by the downward arrowed line 407) throughout the entire readout line 413.

Significantly, between being reset and subsequently readout within the same image frame, the rows of pixels are capable of integration. However, due to the dark environment there is no light for integration (or at least insufficient light for any meaningful amount of integration). Only when the light source is controlled to substantially illuminate during the vertical blanking period will any effective integration or at least a vast majority of the effective integration occur. In other words, outside the actual illumination period, even though the time period is still capable of illumination, there is no effective integration due to an absence of lighting or insufficient lighting. As a result, as shown in FIG. 10 and later in FIG. 11, for a CMOS image sensor that operates using an electrical rolling shutter, global shutter-type images can still be obtained, because all or a vast majority of pixels effectively integrate concurrently over the same period of time (e.g., the effective integration starts when the light source is controlled to provide substantial illumination and effectively stops when the light source is controlled not to provide substantial illumination).

Notice that the resetting of the last row Ry in the reset-readout block 1035-2 for the second video image frame (lower left corner of the parallelogram 1035-2) occurs within the vertical blanking period 1036. In other words, after the vertical blanking period begins, and after illumination with light from the controlled light source potentially begins in the “illumination scheme A” embodiment, the last row of pixels Ry is reset to clear the photodetector for subsequent photocharge production. In some embodiments, in addition to the last row of pixels Ry being reset, anywhere up to about 10% of the other rows of pixels immediately above the last row Ry, may also potentially be reset within the vertical blanking period, depending upon the particular timing of the electrical rolling shutters signals of the embodiment. In other words, typically at least the first 90% of the rows of pixels are not reset after the beginning of the vertical blanking period. Accordingly, the last row Ry (and potentially up to about 10% of the other rows immediately above the last row of pixels) have less time for effective integration than about 90% of the other rows below the first row of pixels that are not reset after substantial illumination commences. This will cause some image distortion, but it generally does not cause excessive image distortion, since generally the integration periods tend to be fairly similar, and since generally only a small percentage (e.g., typically up to about 10%) of the other rows above the last row of pixels have the shorter integration period anyway. Generally, the sooner a row of pixels is reset in a subsequent frame after being read out in a previous frame, the less the difference in integration times will be. If desired, the “illumination scheme B” embodiment as disclosed below may be used to avoid such a difference in the integration times as disclosed above in “illumination scheme A”.

FIG. 11 is a block diagram of reset-readout blocks for consecutive video image frames that illustrate an “illumination scheme B” embodiment in which illumination with light from the light source occurs over potentially an entire and/or potentially any portion of only a post-reset portion 1148 of a vertical blanking period 1136. The post-reset portion 1148 of the terminal blanking period 1136 occurs after reset of the last row of pixels Ry in the first video image frame 1135-1, proceeding after the onset of the vertical blanking period 1136.

In FIG. 11, a first reset-readout block 1135-1 for a first, earlier video image frame and a second reset-readout block 1135-2 for a second, later video image frame are shown. In the “illumination scheme B” embodiment, illumination with light from the light source starts immediately or sometime after the last row of pixels Ry is reset in a second reset-readout block 1135-2 for the video image frame that follows the onset of the vertical blanking period 1136, and the illumination ends at or sometime before the end of the vertical blanking period 1136 (e.g., at or sometime before the first row of pixels is readout during the second reset-readout block 1135-2 for the second video image frame that follows the vertical blanking period). Notice that in the “illumination scheme B” embodiment, illumination with light from the light source is constrained to occur over only the post-reset portion 1148 of the vertical blanking period 1136 at or after the reset of the last row Ry in the second reset-readout block 1135-2. As a result, the “illumination scheme B” embodiment potentially has slightly less time for integration as compared with the “illumination scheme A” embodiment, due to the lower portion of the dark environment 1147 between the beginning of the vertical blanking period (the second dashed line from the top at the right side of FIG. 11) and the resetting of the last row of pixels Ry (the third dashed line from the top at the right side of FIG. 11) occupying the initial, upper portion of the vertical blanking period 1136.

Since in the “illumination scheme B” embodiment, illumination begins at or after the last row of pixels Ry is reset, all of the rows of pixels of the pixel array have the same integration period. This may help to provide slightly less distorted video images. No rows of pixels (e.g., including the last row of pixels Ry) integrate over shorter periods of time than other rows of pixels. Rather, all rows of pixels integrate over the same period of time. Recall from above that this may not be the case for the last set of up to about 10% of the rows of pixels in the “illumination scheme A” embodiment. Accordingly, the “illumination scheme B” embodiment may provide somewhat more accurate or less distorted images than the “illumination scheme A” embodiment. Although, the “illustration scheme A” embodiment may very closely approximate the accuracy of the “illumination scheme B” embodiment for almost all rows of pixels when each row of pixels is reset in a subsequent frame very soon after it is read in a prior frame. In such cases, the “illumination scheme A” embodiment may offer an advantage over “illumination scheme B” embodiment by allowing illumination to be coordinated based on readout signals only (e.g., the readout signal of the last row Ry for the first image frame and the readout signal of the first low R1 for the second image frame, as shown in FIG. 10). In contrast, both the reset signal of the last row Ry for the second image frame and readout signal of the first row R1 for the second image frame can be used to coordinate illumination for the “illumination scheme B embodiment”, as shown in FIG. 11.

For simplicity, a few example illumination scheme embodiments have been described in detail above. Other illumination scheme embodiments are also contemplated. For example, other illumination scheme embodiments may optionally end prior to the end of the vertical blanking period. As another example, other illumination scheme embodiments may use only a central portion of the vertical blanking period not as strictly tied to the timing of the reset and readout lines. As yet another example, other illumination scheme embodiments may extend a bit outside the vertical blanking period in order to trade off some image distortion with increased integration time. Still other embodiments will be apparent to those skilled in the art and having the benefit of the present disclosure.

FIG. 12 is a block diagram of an embodiment of a video image acquisition system 1250 that is operable to acquire global shutter-type video images with a CMOS pixel array that is to use an electrical rolling shutter. The video image acquisition system 1250 includes the CMOS pixel array 1202 that is to use the electrical rolling shutter. A CMOS pixel array support system 1251 is coupled with, and is operable to support the CMOS pixel array 1202. In some embodiments, the video image acquisition system 1251 may represent an endoscope video image acquisition system, although the scope of the invention is not so limited. For example, the video image acquisition system may represent the endoscope video image acquisition system 515 of FIG. 5, or an entirely different one.

The CMOS pixel array support system 1251 includes an apparatus 1225 that is operable to acquire global shutter-type images with the CMOS pixel array using the electrical rolling shutter. The apparatus 1225 includes a clock unit 1252. The clock unit includes a clock signal generator 1253. The clock unit is operable to provide clock signals 1208 to the CMOS pixel array. The apparatus 1225 also includes a video image acquisition control unit 1254. The video image acquisition control unit is operable to provide electrical rolling shutter video image acquisition control signals 1226 to the CMOS pixel array to control the CMOS pixel array to acquire video images using an electrical rolling shutter. The signals for each video image are operable to reset each row of pixels of the CMOS pixel array sequentially, and one row at a time, from a first row to a last row, and then to read each row of pixels of the CMOS pixel array sequentially, and one row at a time, from the first row to the last row. In some embodiments, the signals are such that the resetting of the last row of a given image frame is to be performed before the reading of the first row of the given image frame. The signals define, between each pair of consecutive video images, a vertical blanking period between the reading of the last row of pixels of a prior video image frame and the readout of the first row of pixels of a subsequent video image frame. The video image acquisition control unit is coupled with the clock unit. In some embodiments, the signals may be operable to generate the reset-readout blocks for FIG. 10 and/or FIG. 11. Alternatively, the signals may be operable to generate different reset-readout blocks. The video image acquisition control unit may be implemented in hardware (e.g., circuitry), software, firmware, or a combination thereof.

The apparatus 1225 also includes a light strobing control unit 1256. The light strobing control unit 1256 is coupled with a light source 1222, the video image acquisition control unit 1254, and the clock unit 1252. The light strobing control unit 1256 is operable to provide light strobing control signals to the light source 1222 to control the light source to provide strobed light 1228 to the CMOS pixel array. In some embodiments, the light strobing control unit may be operable to control the light source to provide substantial light during at least a portion of each of the vertical blanking periods, and control the light source not to provide substantial light between: (1) the readout of the first and last rows of pixels during the previous video image frames before the vertical blanking periods; and (2) the readout of the first and last rows of pixels during the subsequent video image frames after the vertical blanking periods. The light strobing control unit may be implemented in hardware (e.g., circuitry), software, firmware, or a combination thereof.

As shown, in some embodiments, the light source 1222 has an on/off control 1255 to allow the light source to be turned on and off multiple or many times in succession by the light strobing control signals in order to provide the strobed light. In some embodiments, the on/off control may switch on and off an LED, laser, lamp, bulb, or other light emission device of the light source. Alternatively, the light from the light source may be blocked/not blocked, reflected/not reflected, diverted/not diverted, or otherwise provided/not provided. As will be mentioned further below, in some embodiments, the light strobing control unit may also include a light intensity control unit (not shown) to control intensity (e.g., to turn up or dim down) of light provided by the light source, although this is not required. Also, in other embodiments, the light source may not be included in the CMOS pixel array support system as previously described.

The video image acquisition control unit 1254, the clock unit 1252, and the light strobing control unit 1256 may coordinate the strobed light 1228, the electrical rolling shutter video image acquisition control signals 1226, and the clock signals 1208, so that illumination occurs only within the vertical blanking periods. In some embodiments, the light strobing control unit 1256 generates the light strobing control signals based on information from the video image acquisition control unit 1254 and/or the clock unit 1252. In some embodiments, the light strobing control signals are timed relative to the electrical rolling shutter video image acquisition control signals 1226 and/or the clock signals 1208. In one embodiment, such as when the image sensor is relatively small, some pads may be used in a multitask fashion for both control and clock signals, although this is not required.

The CMOS pixel array support system 1251 also includes an image processing unit 1257. The image processing unit may be substantially conventional and may process image data signals received from the image sensor in any of various conventional ways that do not limit the scope of the invention. An optional image presentation device 1258 is also shown. The image presentation device may present images from the image processing unit to a user. Examples of suitable image presentation devices include, but are not limited to, display devices, printers, faxes, and other image presentation devices known in the arts. Alternatively, rather than being presented to a user, the images may be stored (e.g., in a memory) or otherwise preserved.

FIG. 13 is a block diagram of an embodiment of a portion of a video image acquisition system 1350 having an automatic exposure control unit 1360. In various embodiments, the portion 1350 may be included in the endoscope video image acquisition system 515 of FIG. 5, the video image acquisition system 1250 of FIG. 12, or a different video image acquisition system.

The portion of the video image acquisition system 1350 includes an image processing unit 1357. The image processing unit may receive image data signals 106 (e.g., from a CMOS pixel array 102 or image sensor 101 of FIG. 1). The image processing unit may process the image data signals. Conventional ways of processing the image data signals are suitable.

The automatic exposure control unit 1360 may receive post-process image data 1306 from the image processing unit 1357. In the illustrated embodiment, the automatic exposure control unit 1360 is shown as being part of a light strobing control unit 1356. Alternatively, the automatic exposure control unit may be separate from the light strobing control unit. The automatic exposure control unit is operable to automatically or autonomously control and adjust the amount of light illumination provided by a light source 1322 based at least in part on the post-process image data 1306. In some embodiments, the automatic exposure control unit may provide feedback control based on information from the already acquired image data. In other embodiments, the automatic exposure control unit may provide feedforward control. In still other embodiments, the automatic exposure control unit may provide both feedback and feedforward control or other types of control. Advantageously, such an automatic exposure control unit may help to improve the quality of images acquired by the video image acquisition system by adjusting the amount of light illumination so that the images are of appropriate brightness, etc.

The automatic exposure control unit 1360 includes an image analysis unit 1361. The image analysis unit 1361 is operable to analyze the received image data 1306. In some embodiments, the analysis may include analyzing exposure-dependent features of the image data that depend on the amount of exposure. A few examples of suitable exposure-dependent features include, but are not limited to, average brightness, brightness distribution, brightness histogram, etc. It will be appreciated by those skilled in the art and having the benefit of the present disclosure that various other features that allow one to determine whether the image is of appropriate brightness may also or alternatively be used. In some embodiments, the image analysis unit and/or the automatic exposure control unit may include a predetermined standard amount of exposure, and may be operable to compare this predetermined standard amount of exposure with the amount of exposure obtained from the received image data. By way of example, the preexisting standard amount of exposure may represent a predetermined desired amount of exposure (e.g., a desired average or minimum brightness of the image).

The automatic exposure control unit 1360 may control the light source 1322 to adjust the amount of light illumination based on the analysis of the image data 1306. In general the amount of light exposure or illumination of the CMOS pixel array in the dark environment depends primarily on (1) the intensity of the light provided by the light source; and (2) the duration of the light provided by the light source. For example, the amount of light exposure or illumination may be approximated by the product of the light intensity times the duration of light. In some embodiments, either or both of the light intensity and/or the duration of the light provided by the light source may be adjusted in order to adjust the amount of illumination or light exposure, as further disclosed below.

As shown, in some embodiments, the automatic exposure control unit includes a light intensity control unit 1362 that is operable to control adjustment of an intensity of the light from the controlled light source during a vertical blanking period for an image frame subsequent to a present image frame, and a light duration control unit 1363 that is operable to control adjustment of duration of the light from the controlled light source during a vertical blanking period for an image frame subsequent to a present image frame. In other embodiments, the automatic exposure control unit may include either, but not both, of these units.

In some embodiments, the duration of light may be controlled by the light duration control unit 1363 exerting control over an on/off control 1355 of the light source 1322, although this is not required. In some embodiments, the light duration control unit 1363 may communicate with the light strobing control unit 1356 to have the light strobing control unit control the on/off control. In some embodiments, the duration of the vertical blanking period may be changed (e.g., increased or decreased). The light duration control unit may also communicate or signal a video image acquisition control unit 1354 and/or a clock unit 1352 to coordinate timing.

In some embodiments, to reduce the duration of the light from the controlled light source 1322, a continuous light duration period may be broken down into a series of shorter light duration periods. FIG. 15 shows two illumination patterns. The light duration period 1510 of the illumination pattern 1501 at the left side of FIG. 15 is continuous. To reduce the duration of light, instead of providing continuous light, the light duration control unit 1363 may control the light source 1322 to provide light in a series of pulses 1530, shown as the illumination pattern 1502 at the right side of FIG. 15. The resulting light duration period 1520 of pattern 1502 contains less illumination time than the light duration period 1510 of pattern 1501. The darkness duration times 1540 and 1550 are substantially the same for both illumination patterns.

Referring again to FIG. 13, in some embodiments, the light intensity may be controlled by the light intensity control unit 1363 controlling intensity control 1364 of the light source 1322. By way of example, this may involve changing a voltage, a current, a power, a combination thereof, or other electrical input to the light source.

To further illustrate certain concepts, consider a few illustrative examples. In one example, if the image data indicates that the image is about half as bright as desired, then the light intensity may be controlled to be about twice as large for a subsequent image frame. As another example, if the image data indicates that the image is about half as bright as desired, then the duration of light may be controlled to be about twice as long for a subsequent image frame. In other examples both duration and intensity together may be changed to achieve the desired brightness. As yet another example, if the brightness is sufficiently close to the desired brightness, but the video images are too choppy as analyzed by the image analysis unit, then the frame rate may be increased. Decreasing the frame rate generally also decreases the vertical blanking period, which in some cases may decrease the duration of light exposure or illumination. If such is the case for the embodiment, then the light intensity may be increased to account for the decrease in light exposure duration so that the amount of light exposure remains approximately the same.

FIG. 14 illustrates both an embodiment of conventional, regular, un-prolonged vertical blanking periods 1465, and an embodiment of prolonged vertical blanking periods 1436. Also disclosed in FIG. 14 is an embodiment of an approach for prolonging the prolonged vertical blanking periods by holding a clock signal 1408 high (i.e., at a constant level) during at least a portion of each of the prolonged vertical blanking periods. The prolonged vertical blanking periods have a longer duration in time than the regular, un-prolonged vertical blanking periods. In various example embodiments, the prolonged vertical blanking periods are at least 110%, at least 120%, at least 150%, at least 200%, or even longer than the regular, un-prolonged vertical blanking periods.

The clock signal for the conventional un-prolonged vertical blanking periods continuously switches between high and low levels at the same clock cycle rate over the entire period of time. In contrast, the clock signal for the prolonged vertical blanking periods does not continuously switch between high and low levels at the same clock cycle rate over the entire period of time shown. Rather, during at least a portion of the vertical blanking periods, the clock signal is held at a constant level (in this case, a high level) for a period of time corresponding to multiple clock cycles. This prolongs the vertical blanking periods. In the illustrated embodiment, the clock signal is held high, although alternatively the clock signal may be held low. Holding the clock signal at the constant level effectively stops the clock and prolongs the vertical blanking periods for the duration that the clock signal is held constant. In some embodiments, in addition to prolonging the vertical blanking periods, the frame rate may be commensurately reduced. In one particular example embodiment, the un-prolonged vertical blanking periods may be approximately 30 ms in a video sequence having approximately 30 frames/sec, whereas the prolonged vertical blanking periods may be approximately 60 ms (i.e., approximately twice as long) in a video sequence having approximately 15 frames/sec (i.e., approximately half the frame rate).

In some embodiments, a video imaging system may include multiple image sensors that may each be used to concurrently acquire images. As used herein, “multiple” image sensors means at least two image sensors. There are various possible reasons for using multiple image sensors to acquire images. As one example, multiple image sensors may be used to acquire images that may be processed to provide holograms, three-dimensional images, or perspective views of an object. For example, two image sensors may be positioned in two different locations so that they each acquire an image of an object from a different viewing perspective or line of site. At least conceptually, this may be similar to two eyes viewing an object from their different positions, which allows three dimensional details to be perceived. As another example, the different image sensors may each be used to acquire an image based on a different color. For example, a first image sensor may acquire an image for a red color component, a second image sensor may acquire an image for a green color component, and a third image sensor may acquire an image for a blue color component. In other examples, the different image sensors may have different attributes, such as, for example, different resolutions, different fields of view, different sensitivities to light, or the like. These are just a few illustrative examples, but still other examples of reasons to use multiple image sensors will be appreciated by those skilled in the art and having the benefit of the present disclosure.

One challenge that presents itself when using multiple image sensors to concurrently to perform operations, for example to acquire images, is that it is generally important to be able to synchronize the image sensors. For example, the process of acquiring a video image frame should generally be synchronized on the different image sensors. For example, commonly the phases of the clock signals (e.g., clock signals 108, 1208, etc.) of the different image sensors should be synchronized, the video image acquisition control signals (e.g., signals 109, 1226, etc.) of the different image sensors should be synchronized. Lack of synchronization may potentially tend to cause inconsistencies or distortions in the images acquired by the different image sensors or otherwise reduce the quality of the images. For example, the image sensors may acquire images of the object at slightly different or offset times instead of at the same time. The need for sufficient synchronization may tend to be even more important when there are temporal changes during the period of image acquisition. One example of such a temporal change is changing light conditions, such as, for example, when a light is being strobed as disclosed above in conjunction with acquiring global shutter-type video images with a CMOS pixel array using a rolling shutter. Another example of such a temporal change is movement of the image sensors during the period of image acquisition, such as, for example, in the case of an endoscope being advanced or moved within a patient.

One possible approach that may be used to synchronize image sensors is to use a dedicated synchronization signal specifically designed to synchronize the image sensors. For example, a controller, microprocessor, or base station may generate and provide dedicated synchronization signals to each of the image sensors at the same time. Typically, the image sensors may include dedicated electrical contact pads, or other dedicated electrical contact structures, to receive these synchronization signals. Commonly, these dedicated electrical contact structures may be used specifically and only for the receipt of these synchronization signals. The image sensors may be operable to use the synchronization signals to synchronize their clock signals and image acquisition processes. The synchronization signals may be provided to the internal clock dividers of each image sensor. Typically, the image sensors will each include a phase-locked loop (PLL) operable to synchronize clock signals provided to the different image sensors so that the phases of the clock signals start at the same time for each image sensor. Throughout such a synchronization process the clock signals provided to the image sensors typically keep running at a constant or invariable rate or frequency. Keeping the clock signals invariant throughout this process is generally consistent with conventional wisdom that the clock signals typically should not be altered.

However, there are various potential drawbacks with such an approach for synchronizing image sensors by using dedicated synchronization signals. For one thing, as mentioned above, typically, the image sensors may include dedicated electrical contact structures (e.g., contact pads) to receive these synchronization signals. However, these dedicated pads or other electrical contact structures generally occupy additional space on the image sensors. Also, additional electrical lines or other conductive paths generally need to be made to these pads or other electrical contact structures. Accordingly, these dedicated electrical contact structures may tend to increase the size, complexity, and manufacturing cost of providing the image sensors. Similarly, each image sensor typically includes a PLL. However, such PLLs also generally occupy additional space on the image sensors. Moreover, they tend to consume additional power. Accordingly, these PLLs may tend to increase the size, complexity, power consumption, and manufacturing cost of providing the image sensors. In some implementations, such as, for example, when the image sensors are to be inserted into endoscopes that are desired to be very small, it is desirable to make the image sensors very small. Especially when manufacturing very small image sensors, being able to omit the additional dedicated electrical contact structures and/or the PLLs may help to allow reductions in the size of the image sensors.

Some embodiments of the invention pertain to new and useful methods and apparatus to synchronize two or more image sensors. In some embodiments, a clock signal may be used to synchronize two or more image sensors. Advantageously, in some embodiments, through the use of the clock signal there may be no need for an additional dedicated synchronization signal. Rather, the clock signal itself may be used to synchronize the multiple image sensors. In some embodiments, the clock signals that are used to synchronize the image sensors may be provided through conventional clock signal pads or other electrical contact structures (e.g., CLK pads) typically already present in the image sensors. That is, the clock signal pads or other electrical contact structures may be used both to deliver the clock signals to the image sensors and in some embodiments may also be used to synchronize the image sensors. Advantageously, in some embodiments, there may be no need for additional dedicated electrical contact structures. Advantageously, avoiding such additional dedicated electrical contact structures may help to reduce the size, complexity, and/or manufacturing cost of providing the image sensors. In some embodiments, there may be no need for each image sensor to include a PLL or other clock phase locking circuit for the purpose of synchronize the clock signals. Advantageously, omitting such PLLs may help to reduce the size, power consumption, and/or manufacturing cost of providing the image sensors. Using the clock signal to synchronize the image sensors may be especially beneficial when it is desired to make one or more of the image sensors very small, since it may help to allow omitting the additional dedicated electrical contact structures and/or PLLs, which may help to facilitate reduction in the size of the image sensors. However, the scope of the invention is not limited to such small image sensors.

FIG. 16 is a block diagram of an embodiment of multiple image sensor video image acquisition system 1650 that is operable to use a synchronization clock signal 1671 to synchronize a first image sensor 1601-1 and a second image sensor 1601-2. The first and second image sensors may either be the same or different. The system includes a clock control unit 1652 that includes a synchronization control unit 1670. The synchronization control unit is operable to cause the clock control unit to output the synchronization clock signal 1671. The synchronization control unit may be implemented in hardware (e.g., circuitry), firmware, software, or a combination thereof. The synchronization clock signal is a clock signal that is also operable to be used for synchronization. The synchronization clock signal is provided to a clock divider 1672. The clock divider is operable to divide the synchronization clock signal into a first copy of the synchronization clock signal provided on a first cock signal delivery line 1673-1 to the first image sensor 1601-1 and a second copy of the synchronization clock signal provided on a second cock signal delivery line 1673-2 to the second image sensor 1601-2. In the illustrated embodiment, the clock divider is external to the clock control unit, although in another embodiment the clock divider may be internal to the clock control unit. The first image sensor has a first clock pad or other electrical contact structure 1674-1 to receive the synchronization clock signal. Likewise, the second image sensor has a second clock pad or other electrical contact structure 1674-2 to receive the synchronization clock signal. In some embodiments, the first and second clock pads are those that receive the clock signals for the corresponding image sensors and in some embodiments are also used for synchronization.

The first image sensor includes a first control and logic unit 1603-1 coupled with the first clock pad 1674-1 to receive the synchronization clock signal. Likewise, the second image sensor includes a second control and logic unit 1603-2 coupled with the second clock pad 1674-2 to receive the synchronization clock signal. The first control and logic unit 1603-1 includes a first synchronization unit 1675-1, and the second control and logic unit includes a second synchronization unit 1675-2. The first and second synchronization units are each operable to synchronize their corresponding image sensors using the synchronization clock signal 1671. The first and second synchronization units may be implemented in hardware (e.g., circuitry), firmware, software, or a combination thereof. In some embodiments, the first and second synchronization units may each be operable to synchronize their corresponding image sensors by detecting or identifying an end of a prolonged constant clock signal. In one embodiment, the synchronization units may be operable to synchronize their respective image sensors by detecting or identifying a falling edge of a prolonged high cycle of a prolonged high synchronization clock signal. In another embodiment, the synchronization units may be operable to synchronize their respective image sensors by detecting or identifying a rising edge of a prolonged low cycle of a prolonged low synchronization clock signal. In some embodiments, the prolonged high or prolonged low cycles may be used to provide prolonged vertical blanking periods (e.g., as described above in conjunction with the prolonged vertical blanking period 1436 of FIG. 14), although the scope of the invention is not so limited.

FIG. 17A is a diagram illustrating an example embodiment of a prolonged high synchronization clock signal 1771A. The prolonged high synchronization clock signal includes a prolonged high cycle 1776 where the clock signal is held at a high level for a period of time corresponding to multiple regular clock cycles. A falling edge 1777 of the prolonged high cycle 1776 may be identified or detected and used for synchronization. For example, an image acquisition process may commence upon detection or identification of the falling edge 1777. Thereafter, the image acquisition process may proceed according to the subsequent regular clock cycles.

FIG. 17B is a diagram illustrating an example embodiment of a prolonged low synchronization clock signal 1771B. The prolonged low synchronization clock signal includes a prolonged low cycle 1778 where the clock signal is held at a low level for a period of time corresponding to multiple regular clock cycles. A rising edge 1779 of the prolonged high cycle 1778 may be identified or detected and used for synchronization. For example, an image acquisition process may commence upon detection or identification of the rising edge 1779. Thereafter, the image acquisition process may proceed according to the subsequent regular clock cycles.

FIG. 18 is a block diagram of an embodiment of an image sensor 1801 having an example of a suitable set of pads or other electrical contact structures. As shown, in some embodiments, the image sensor may have one or more power pads 1880 to receive power signals, a ground pad 1881 to be coupled with ground, one or more data pads 1882 to provide data signals, one or more control pads 1883 to exchange control signals, a clock pad 1874 to receive synchronization clock signals. As shown, in some embodiments, the image sensor may omit a separate dedicated synchronization signal pad 1884 of the type conventionally used to receive a dedicated synchronization signal (i.e., not the synchronization clock signals disclosed herein). As further shown, in some embodiments, the image sensor may omit a phase lock loop (PLL) 1885 of the type conventionally used to synchronize the image sensor to a clock signal. As previously mentioned, the ability to omit such components may potentially help to reduce the size, complexity, power consumption, and manufacturing cost of the image sensor. In other embodiments, the image sensors disclosed herein may optionally have a PLL (e.g., to help improve synchronization relative to the clock signal). Yet in other embodiments, in addition to removing synchronization signal pad 1884 and/or PLL 1885 and having synchronization function performed through clock pad 1874, the data pad 1882 may be further combined with the control pad 1883 to reduce the total number of pads to four (not shown in FIG. 18). Alternatively, the clock pad 1874 may be further tasked to handle control and/or data signals. The resulting number of pads will therefore be kept at four. Detailed specification of combining clock pad 1874, data pad 1882, and/or control pad 1883 may be found in U.S. Patent Application Publications 2013/0264465 and 2013/0264466, which are whereby incorporated by reference in their entirety.

FIG. 19 is a block diagram of an embodiment of a multiple image sensor video image acquisition system 1950 that is operable to acquire global shutter-type video images with a CMOS pixel array that is to use an electrical rolling shutter. The system 1950 has certain similarities to the single image sensor system 1250 of FIG. 12. Unless otherwise specified, the corresponding named components in FIGS. 12 and 19 may optionally have the same or similar characteristics. To avoid obscuring the description, the different and/or additional characteristics of the system 1950 will primarily be described without repeating all of the characteristics which may optionally be the same or similar.

The multiple image sensor video image acquisition system 1950 has a first image sensor 1901-1 and a second image sensor 1901-2. As shown, the first image sensor may have a CMOS pixel array to use an electrical rolling shutter. In the illustrated embodiment, the second image sensor may also have a CMOS pixel array to use an electrical rolling shutter, although this is not required. In another embodiment, only one but not both of the image sensors may use the approaches described herein to acquire global shutter-type video images with pixel arrays using rolling shutters. In the illustrated embodiment, strobed light 1228 may be provided to both the first and second image sensors. Likewise, electrical rolling-shutter video image acquisition control signals 1226 may also be provided to both the first and second image sensors. In some embodiments, synchronization clock signals 1971 may be provided to both the image sensors. By way of example, in some embodiments, the synchronization clock signal may represent the clock signal 1408 with the prolonged vertical blanking period 1436 of FIG. 14, although the scope of the invention is not so limited. As shown, a clock unit 1952 may include a synchronization control unit 1970 (e.g., similar to the unit 1670) to control the generation of the synchronization clock signals. As further shown, the first image sensor may include a clock pad 1974 to receive the synchronization clock signals. This may also be the case for the second image sensor. The first image sensor may have a synchronization unit 1975 to synchronize the first image sensor based on the synchronization clock signal. This may be done substantially as previously described.

In some embodiments, the synchronization approaches shown and described for FIGS. 16-19 may optionally be used with the other approaches described herein to acquire global shutter-type video images with a pixel array using an electrical rolling shutter. For example, the apparatus of any of FIG. 16, 18, or 19 may optionally be incorporated into the apparatus of any of FIG. 5 or 12. Likewise, the apparatus of any of FIG. 16, 18, or 19 may optionally be used in the methods of any of FIG. 7, 8, or 9. Components, features, and optional details for these approaches may optionally also be included in the approaches of FIGS. 16-19. Alternatively, the synchronization approaches shown and described for FIGS. 16-19 may optionally be used without the other approaches described herein.

FIGS. 20A and 20B are diagrams of an endoscope 2000 including an image sensor 2020, in accordance with an embodiment of the disclosure. Endoscope tip 2005 is for inserting, often into a cavity of a subject of an endoscope examination, to provide imaging data. In FIG. 20A, image sensor 2020 is disposed on endoscope tip 2005. FIG. 20A also illustrates host controller 2030 coupled to image sensor 2020 via four terminals 2035. Image sensor 2020 may include a CMOS pixel array to use electrical rolling shutter, as disclosed above as CMOS pixel arrays 502 and 1202 in FIGS. 5 and 12 respectively. Host controller 2030 may be any of the previously discussed controllers including endoscope base station 516 in FIG. 5, and apparatus 1225 and unit 1251 in FIG. 12.

FIG. 20B is a front view of endoscope tip 2005 that includes lights 2010, and accessories 2015 and 2025. Endoscope tip 2005 may be used in the medical field or otherwise. Accessories 2015 and 2025 may include suction or forceps utilities. The reduction in the number of terminals 2035 included on image sensor 2020 may allow for the overall size of image sensor 2020 to be reduced, and in turn, the overall size of endoscope tip 2005 may be reduced. In addition, a reduced size image sensor 2020 may allow for improved, larger, or additional accessories to fit within endoscope tip 2005. Any of these improvements may increase the success rate of the action being performed with the endoscope (such as surgery).

FIG. 20C is a top view of an endoscope tip 2007 that includes image sensor 2020 and lights 2012. This is a type of endoscope that is used for diagnostic purposes, thus it includes relatively large areas for lighting. In this embodiment, four LED lights 2012 surround image sensor 2020. In another embodiment, shown in FIG. 20D, an area around image sensor 2020 of endoscope tip 2009 is occupied by a multitude of optic fibers 2014, which is used to provide lighting at endoscope tip 2009.

Embodiments have been described in conjunction with endoscope video imaging systems. However, the scope of the invention is not so limited. Other embodiments are suitable for boroscopes, hydraulic pigs, other monitoring probes, and other inspection devices for engine, industrial, pipeline, and other applications. There is no requirement of two-part form factor in which a CMOS pixel array is separated from (e.g., connected by cable(s) to) a support system. Other embodiments may be used in single form factor video image acquisition systems, such as, for example, standard digital cameras. Embodiments have been described above in conjunction with a moving CMOS pixel array and a stationary object being imaged. However, other embodiments are applicable to a stationary CMOS pixel array and a moving object being imaged. Accordingly, embodiments pertain to a wide variety of different types of devices having CMOS pixel arrays that are to be used in dark or relatively dark environments when there is relative movement between the CMOS pixel array and an object being imaged (e.g., the CMOS pixel array and/or the object is moving).

In the description and claims, the terms “coupled” and/or “connected,” along with their derivatives, have be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other.

In the description and claims, the term “logic” has been used. As used herein, the term logic may include hardware (e.g., circuitry), firmware, software (e.g., instructions stored on a tangible storage medium), or various combinations thereof. Examples of logic include integrated circuitry, application specific integrated circuits, analog circuits, digital circuits, programmed logic devices, memory including instructions, etc. In some embodiments, the logic may include at least some circuitry (e.g., transistors, active circuit elements, passive circuit elements, integrated circuitry, etc.).

In the description above, specific details have been set forth in order to provide a thorough understanding of the embodiments. However, other embodiments may be practiced without some of these specific details. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. All equivalent relationships to those illustrated in the drawings and described in the specification are encompassed within embodiments. In other instances, well-known circuits, structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description.

Where multiple components have been shown and described, in some cases, these multiple components may optionally be integrated into one component. Where a single component has been shown and described, in some cases, this single component may be separated or divided into two or more components. In the illustrations lines (e.g., arrows) are used to show connections and couplings.

Certain methods disclosed herein have been shown and described in a basic form, although operations may optionally be added to and/or removed from the methods. In addition, a particular order of the operations may have been shown and/or described, although alternate embodiments may perform certain operations in different order, combine certain operations, overlap certain operations, etc.

One or more embodiments include an article of manufacture (e.g., a computer program product) that includes a machine-readable medium. The medium may include a mechanism that provides (e.g. stores) information in a form that is readable by a machine. The machine-readable medium may provide, or have stored thereon, a sequence of instructions that if executed by the machine causes or results in the machine performing operations and/or methods disclosed herein. Examples of suitable machines include, but are not limited to, endoscope base stations, video image acquisition systems, digital video cameras, and other video image acquisition systems having CMOS pixel arrays, computer systems, electronic devices having processors, etc.

In one embodiment, the machine-readable medium may include a tangible non-transitory machine-readable storage media. For example, the tangible non-transitory machine-readable storage media may include a floppy diskette, an optical storage medium, an optical disk, a CD-ROM, a magnetic disk, a magneto-optical disk, a read only memory (ROM), a programmable ROM (PROM), an erasable-and-programmable ROM (EPROM), an electrically-erasable-and-programmable ROM (EEPROM), a random access memory (RAM), a static-RAM (SRAM), a dynamic-RAM (DRAM), a Flash memory, a phase-change memory, or a combinations thereof. The tangible medium may include one or more solid materials, such as, for example, a semiconductor material, a phase change material, a magnetic material, etc.

Reference throughout this specification to “one embodiment,” “an embodiment,” “one or more embodiments,” “some embodiments,” for example, indicates that a particular feature may be included in the practice of the invention but is not necessarily required to be. Similarly, in the description various features are sometimes grouped together in a single embodiment, Figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention. 

1. A multiple image sensor image acquisition system comprising: a clock control unit to generate a synchronization clock signal, the synchronization clock signal having a prolonged constant cycle during which the synchronization clock signal is held at a constant level for a period of time corresponding to multiple clock cycles; a first image sensor coupled with the clock control unit to receive the synchronization clock signal, the first image sensor having a first synchronization unit that is operable to synchronize image acquisition for the first image sensor based on detection of an end of the prolonged constant cycle; and a second image sensor coupled with the clock control unit to receive the synchronization clock signal, the second image sensor having a second synchronization unit that is operable to synchronize image acquisition for the second image sensor based on detection of the end of the prolonged constant cycle.
 2. The system of claim 1, wherein the prolonged constant cycle comprises a prolonged high cycle during which the synchronization clock signal is held high for the period of time.
 3. The system of claim 2, wherein the first synchronization unit is operable to synchronize image acquisition for the first image sensor based on detection of a falling edge of the prolonged high cycle.
 4. The system of claim 1, wherein the prolonged constant cycle comprises a prolonged low cycle during which the synchronization clock signal is held low for the period of time.
 5. The system of claim 4, wherein the first synchronization unit is operable to synchronize image acquisition for the first image sensor based on detection of a rising edge of the prolonged low cycle.
 6. The system of claim 1, wherein the first image sensor has a clock electrical contact structure to receive the synchronization clock signal, wherein the clock electrical contact structure is used to receive clock signals used by the first image sensor during the image acquisition.
 7. The system of claim 1, further comprising a light control unit to turn a light on during the prolonged constant cycle and to turn the light off at the end of the prolonged constant cycle.
 8. The system of claim 1, wherein the prolonged constant cycle corresponds in time to a vertical blanking period of the first image sensor.
 9. The system of claim 8, wherein the prolonged constant cycle is used to prolong the vertical blanking period of the first image sensor.
 10. The system of claim 1, wherein the first image sensor does not have a phase lock loop (PLL) to synchronize image acquisition for the first image sensor.
 11. The system of claim 1, wherein the first image sensor does not have a dedicated synchronization electrical contact structure to receive a dedicated synchronization signal.
 12. The system of claim 1, wherein the first and second image sensors are disposed within a distal end of an endoscope.
 13. A method of synchronizing image sensors comprising: generating a synchronization clock signal, the synchronization clock signal having a prolonged constant cycle during which the synchronization clock signal is held at a constant level for a period of time corresponding to multiple clock cycles; synchronizing image acquisition for a first image sensor based on detection of an end of the prolonged constant cycle; and synchronizing image acquisition for a second image sensor based on detection of the end of the prolonged constant cycle.
 14. The method of claim 13, wherein generating comprises generating the synchronization clock signal having a prolonged high cycle during which the synchronization clock signal is held high for the period of time, and wherein synchronizing the image acquisition for the first image sensor is based on detection of a falling edge of the prolonged high cycle.
 15. The method of claim 13, wherein generating comprises generating the synchronization clock signal having a prolonged low cycle during which the synchronization clock signal is held low for the period of time, and wherein synchronizing the image acquisition for the first image sensor is based on detection of a rising edge of the prolonged low cycle.
 16. The method of claim 13, further comprising: receiving the synchronization clock signal on a clock electrical contact structure of the first image sensor; and receiving clock signals on the clock electrical contact structure of the first image sensor during the image acquisition.
 17. The method of claim 13, further comprising: turning a light on during the prolonged constant cycle; and turning the light off at the end of the prolonged constant cycle.
 18. The method of claim 13, wherein generating comprises generating the synchronization clock signal having the prolonged constant cycle which corresponds in time to a vertical blanking period of the first image sensor.
 19. The method of claim 13, wherein synchronizing the image acquisition for the first image sensor is performed without using a phase lock loop (PLL). 